Semiconductor devices used in high performance applications are often compound semiconductor devices. Group III-V semiconductors, such as GaAs and GaN, are typical compound semiconductor materials used in high performance applications. GaAs devices are often used in high voltage, high power, and high frequency products. Volume production for more widespread application of such products has called for compound semiconductor device products to be small in size and cost competitive.
Interest in fabrication cost reduction has led to low cost packaging of compound semiconductor devices. High power and high frequency devices were traditionally packaged in hermetically sealed, cavity-type packages. Such packages usually provide adequate moisture protection at the chip level. Cheaper, plastic packaging does not provide a good moisture barrier. Plastic packages may thus leave moisture protection to the chip itself.
Ohmic fences have been used to provide moisture protection in GaAs devices and products with plastic packaging. Unfortunately, circuit layouts with ohmic fences were unable to prevent chip level failures during conventional temperature, humidity, bias (THB) testing. Humidity penetrating through the plastic packaging led to the failures.
GaAs devices are also at risk of fracture during the manufacturing process due to the brittle nature of the die substrate. The die is especially susceptible once the wafer is thinned. Die fractures are commonly a result of issues at singulation (wafer saw), tape transfer, die bond, wire bond, package singulation, test, or any other physical handling step. Unfortunately, not all fractured devices fail an electrical test. For example, a fracture may not cross a circuit component capable of causing a failure. In other cases, the fracture may occur in the GaAs substrate, but the metallization above does not separate. The semi-insulating nature of the GaAs substrate may allow a fracture to go undetected.
Die fractures typically result in an unpredictable rate of failure at the customer site. The fracture often increases during a device reflow process for attachment to a circuit board. The increased fracture can then cause an electrical failure at the board level, even though the component passed a final test. In other scenarios, the fracture grows large enough to cause a failure only after use in the field.
Attempts to prevent customers from receiving fractured devices have included rigorous inspections at each of the various process steps likely to cause fractures. Such inspections are often not effective at detecting all defects. The inspections may also not be a viable manufacturing technique for multiple reasons. For example, the use of epoxy die attach materials and overmolding may inhibit any further inspections after those process steps.